1. Technical Field
This invention relates to intake and exhaust port liners for the head of an internal combustion (IC) engine, and more particularly, to such port liners formed of a fiber reinforced ceramic matrix composite (FRCMC) material and methods for making them.
2. Background Art
Exhaust port liners in an internal combustion (IC) engine are used to conserve the residual heat of exhaust gases traveling through them from the cylinder to the exhaust manifold. Conserving the residual heat of the exhaust gases has many advantages. First, it reduces the amount of heat transferred to the head and block of the engine. The heat transferred to the engine must be dissipated, typically by employing water passages which circulate coolant from a radiator. The more heat transferred, the larger the water passages have to be in order to cool the engine. Larger passages mean a bigger, heavier engine, as well as a bigger radiator. The net result is to increase the size and weight of a engine which is disadvantageous for several reasons including performance and fuel economy. Thus, reducing the transfer of heat from the exhaust gases to the engine allows for a smaller, more fuel efficient engine.
The performance and fuel economy of an engine can also be improved by increasing the temperature of the combustion chamber (i.e. the cylinder). This improvement results because the higher chamber temperatures cause a more complete burning of the fuel. Therefore, more energy is transferred and less fuel is required to drive the engine. However, in the past this has not been possible because, among other things, the cooling system of the engine would have to be upgraded as described above, thereby adding to its size and weight. By reducing the heat transfer to the head and block of the engine, a higher combustion chamber temperature could be maintained without the need to upgrade the cooling system.
Another key advantage of conserving the residual heat of the exhaust gases traveling through an exhaust port is in controlling engine emissions. The heat residue in the exhaust gases causes further oxidation of pollution causing unburned fuel components. In addition, current emission control devices, such as a catalytic converter, which are typically disposed downstream of the ports in a vehicle's exhaust system, require hot exhaust gases to perform efficiently. Essentially, these devices operate by reacting with the exhaust gases to cause further oxidation of the unburned fuel components. A high temperature is required to facilitate this reaction. Therefore, the more retained heat in the exhaust gas, the more effective the emission control device is at reducing pollution levels from the engine.
The use of intake port liners in an IC engine also has advantages. As the vaporized fuel/air mixture travels through an un-lined intake port of an IC engine, heat is transferred from the engine head to the fuel-air mixture causing it to expand and increase in pressure. The fuel-air mixture is ultimately compressed in the cylinder of the engine. As such, the aforementioned expansion and increase in pressure requires that more energy be expended to compress the vaporized fuel/air mixture. Therefore, the transfer of heat to the fuel-air mixture as it passes through the intake port, decreases the efficiency of the engine. The intake port liners are designed to thermally insulate at least a portion of the intake ports of the engine, thereby decreasing the transfer of heat to the vaporized fuel and improving the efficiency of the engine.
One typical port liner used in the past is a monolithic ceramic liner. This type of liner is inserted into the exhaust port bore of an engine to thermally insulate the head and block from the exhaust gases flowing through the exhaust ports. In addition, monolithic ceramic port liners are inserted into the intake port bores of an IC engine to thermally insulate vaporized fuel from the head and block of the engine. These monolithic ceramic port liners are usually made from a sintered powderized ceramic material, such as alumina or zirconium. The ceramic port liners can withstand very high temperatures (e.g. &gt;2000.degree. F.) and provide an excellent thermal insulative effect so as to minimize the transfer of heat. However, monolithic ceramic structures are very limited in the shapes in which they can be formed. For example, a straight cylindrical shape is possible, but not one having a bend. This causes some problems in that the port structures of an IC engine usually have complex paths, often including a severe bend. As the monolithic ceramic liner is typically limited to a straight cylindrical shape, they often take the form of short inserts insulating the straight portions of the port. However, the bend area in left uninsulated resulting in a local "hot spot" in the head which requires the use of larger cooling passages in that region, and all the disadvantages associated with the larger cooling requirements. In addition, local "hot spots" can create a mismatch in the thermal expansion of the head and cause it to crack. This situation is made worse by the fact that the bend in the exhaust port corresponds to the location where the hot exhaust gases directly impinge on the walls of the port, and is a site of increase turbulence in the flow of the exhaust gases. Thus, the bend is the site where the heat transfer is at its greatest, and so the place where insulation is most needed.
In addition, monolithic ceramic port liners tend to be porous and brittle. These parts are easily broken or cracked when impacted, or otherwise subjected to even moderate forces. They are also strain intolerant and cannot be deflected more than 0.09 percent without being fractured. This makes inserting the monolithic ceramic port liners into the ports a delicate operation which can easily lead to broken pieces. In addition, thermal expansion and contraction of the head around the relatively "expansion-stable" ceramic liner must be considered. For example, if the head were to contract due to being exposed to cold, care must be taken to ensure there is enough clearance between the walls if the port and the liner to prevent excessive strain being placed on the liner. If the strain is too much, the liner can fracture and fail.
As mentioned above, monolithic ceramic liners are designed to be inserted into a port bore, which can be a very difficult task owing to the delicate nature of the ceramic material. Thus, the insertion process can be costly and time consuming. One desirable option would involve casting the liner directly into the head when it is formed. This would eliminate the insertion step altogether. However, monolithic ceramic liners tend to be thermal shock sensitive. The sudden change in temperature when molten metal used to cast the head is poured around them would cause the liner to fracture and fail. Consequently, casting monolithic ceramic port liners in place is not a viable option without using some intermediate material between the liner and the molten metal.
Accordingly, there is a need for a port liner which exhibits high temperature resistance and thermal insulative capability, but which can be formed into complex shapes so as to insulate the entire length of a port bore. In addition, there is a need for such a port liner which is ductile and less susceptible to fracturing due to handling or thermally-induced strains, than a monolithic ceramic liner. This port liner should also be able to be cast in place when molding the head of the engine.
Wherefore, it is an object of the present invention to provide a port liner which can be formed into practically any shape and size desired, so as to be made to conform to the shape of the port bore in the head of a IC engine.
Wherefore, it is another object of the present invention to provide a port liner which is ductile and fracture resistant, and capable of withstanding thermally-induced strains.
Wherefore, it is still another object of the invention to provide a port liner which can withstand the thermal shock of being cast in place while forming the head of an IC engine.